1. Field of the Invention
The present invention relates to a method of analyzing mass spectra obtained by a multi-turn time-of-flight mass spectrometer.
2. Description of Related Art
The resolution R of a time-of-flight mass spectrometer is given by                     R        =                  T          dT                                    (        1        )            where dT is the time width of each ion peak and T is the flight time. Accordingly, if the time width dT is constant, the resolution can be increased by increasing the flight time T. To achieve this object, multi-turn time-of-flight mass spectrometers in which increased flight distances are used to improve the resolution R have been proposed in the field of mass spectrometry using time-of-flight mass spectrometers.
A multi-turn time-of-flight mass spectrometer can increase the flight distance of ions while avoiding increase in size of the instrument. Therefore, the small instrument can obtain mass spectra with high mass resolution. To increase the flight time T, it is necessary to increase the flight distance. This, in turn, increases the size of the instrument. Accordingly, it has been contemplated to increase the flight distance by making ions rotate along the same trajectory many times. FIG. 1 shows one example of such an instrument.
Multi-turn time-of-flight mass spectrometers are disclosed in the following publications: Japanese Patent Laid-Open No. H11-135060; Japanese Patent Laid-Open No. H11-135061; and Japanese Patent Laid-Open No. H11-195398.
With this instrument, however, as ions make many revolutions, faster ions overrun slower ions on the circulating trajectory. After this phenomenon, peaks of ions having undergone different numbers of turns are mixed on the obtained flight time spectrum. This spectrum is hereinafter referred to as a heterogeneous turn number spectrum. On the other hand, an ordinary time-of-flight spectrum is referred to as a single turn number spectrum. In a heterogeneous turn number spectrum, the numbers of turns of ions giving peaks are unknown in cases where there are peaks of unknown masses. Therefore, it is difficult to obtain a mass spectrum from a time-of-flight spectrum.
Accordingly, in order that ions make many turns and a single turn number spectrum be obtained, the velocity range of ions to be measured simultaneously may be restricted to prevent overrunning of ions spatially.
In this method, however, the mass range of ions that can be measured simultaneously is restricted. Where ions in a wide range should be measured, it is necessary to splice together many single turn number spectra. Consequently, the sensitivity is substantially deteriorated. This is detrimental to the measuring time.